What should I connect with the Emission notch in the Principle PSDF (Blender 2.8)
All the connections are color coded, the "Emission" slot on the "Principled BSDF" shader for example is yellow and stands for "Color", while grey connections are usually for sources that deliver values between 0 and 1. (usually because they do not complain, more about that later)

The blueish connections are for vectors, means a triplet of values from 0.0 to 1.0, here you can also connect color values as they are delivered as triplets RGB as well.

And then you have the green connections, which are either the output of a shader node or the input for the "Material Output" node.

You can connect colors to grey nodes, but it is usually better to convert the values to the correct range, be it by "Color Ramp" node or "RGB to BW" node. "Image Texture" nodes with their Color Space set to "Non-Color" or "Raw" also deliver values from 0 to 1, and should therefor change their output to grey if you ask me, but that's something you for now just have to keep in mind. Because "Image Texture" nodes set to for example "sRGB" default would represent a grey-scale image different then if you set the node to "Raw", since it would convert the data into another Color Space, which is not wanted. You can test it by using a height map image and changing the Color Space from "Raw" as it should be, to "sRGB".

As mentioned already, you can use colors as input for a vector connection, but here it's up to you to know what you are doing. One example would be that you can use the "Mix Color" node with the "Factor" set to 1.0 in the same way the "Vector Math" node would work. Both give the same result but i'd suggest to try and stick with the color coded connections to be on the safe side.

Since the Emission connection is yellow, a color, you can connect "Image Texture" nodes as well as "RGB" nodes, but here a little tip:

If you want to have a really shiny "Eevee" "Bloom" effect, connect a "Combine XYZ" node (vector) BUT here comes the thing.. you can enter a value like 500 for X,Y or Z and it will literally bloom your mind ;)
As you can use the XYZ as RGB you can compared to a "RGB" node go over the 1.0 value and thus create cool neon effects.